Immediately Dangerous to Life or Health (IDLH)

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tox exposure guidelines (1)

Exposure Limits for Chemical Mixtures

Immediately Dangerous to Life or Health (IDLH). In the May 1987 "NIOSH Respirator
Decision Logic", IDLH is defined as a condition "that poses a threat of exposure to airborne
contaminants when that exposure is likely to cause death or immediate or delayed permanent
adverse health effects or prevent escape from such an environment. The purpose of establishing
an IDLH exposure level is to ensure that the worker can escape from a given contaminated
environment in the event of failure of the respiratory protection equipment.
Other organizations, such as ANSI, OSHA, and the Mine Safety and Health Administration
(MSHA), have defined IDLH similarly. It is accepted by all of these groups that IDLH
conditions include not only toxic concentrations of contaminants, but also oxygen-deficient
atmospheres and explosive, or near-explosive (above, at, or near the lower explosive limits),
environments.
At hazardous material incidents, IDLH concentrations should be assumed to represent
concentrations above which only workers wearing respirators that provide the maximum
protection (i.e., a positive-pressure, full-facepiece, self-contained breathing apparatus [SCBA] or
a combination positive-pressure, full-facepiece, supplied-air respirator with positive-pressure
escape SCBA) are permitted. Specific IDLH concentrations values for many substances can be
found in the NIOSH "Pocket Guide to Chemical Hazards." Guidelines for potentially explosive,
oxygen deficient, or radioactive environments can be found in the U.S. EPA "Standard Operating
Safety Guidelines" and the NIOSH/OSHA/USCG/EPA Occupational Safety and Health
Guidance Manual for Hazardous Waste Site Activities.
Exposure Limits for Chemical Mixtures
The exposure limits that have been discussed are based upon exposure to single chemicals. Since
many exposures include more than one chemical, values are adjusted to account for the
combination. When the effects of the exposure are considered to be additive, a formula can be
used to determine whether total exposure exceeds the limits. The calculation used is:
E
m
= (C
1
/L
1
+ C
2
/L
2
) + ... (C
n
/L
n
)
where:
E
m
is the equivalent exposure for the mixture.
C is the concentration of a particular contaminant.
L is the exposure limit for that substance.
The value of E
m
should not exceed unity (1).
An example using this calculation would be as follows:

(Created 12/02)
UNL Environmental Health and Safety · (402) 472-4925 · http://ehs.unl.edu
Chemical A : C = 200 ppm, L = 750 ppm
Chemical B : C = 100 ppm, L = 500 ppm
Chemical C : C = 50 ppm, L = 200 ppm
E
m
= 200/750 + 100/500 + 50/200
E
m
= 0.27 + 0.20 + 0.25
E
m
= 0.72
Since E
m
is less than unity, the exposure combination is within acceptable limits.
This calculation applies to chemicals where the effects are the same and are additive. If the
combination is not additive, the calculation is not appropriate.

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